Define systems 150 volts or less

[pfalcon]

So, using that logic, under normal circumstances, nothing special or off the wall, there would be no such thing as an AC "system", meaning a distribution panel, under 150 volts. I have never seen an AC panel with only one buss.

I appreciate everyone's help and advice on this, however, de-energizing the system is a different topic. The original question was to define a "system" of less than 150 volts. Would this be phase to phase or phase to ground? An example of a system is a main AC 120/208 distribution panel feeding multiple branch circuits. So would you consider this under 150 volts or over 150 volts. If anyone thinks it is phase to phase can you give an example, again normal everyday stuff, nothing off the wall, of an AC system less than 150 volts.

After talking with another electrician, he seems to recall the instructor in his NFPA 70e class telling him that if it is not stated phase to phase then it is assumed phase to ground. However he cannot find any documentation to back that up.

Our typical plant panel is considered a 480VAC hazard even when the disconnect and a top mounted transformer are the only devices using 480VAC.

Common to our plant:
Main panel is 480VAC three-phase :: 480VAC system
Sub panel fed 120VAC single-phase from the main panel :: 120VAC system
Sub panel fed 24VDC from either a 480VAC main or 120VAC secondary :: 24VDC system

And just stumbled on it cause I decided to look:
NFPA 70E : 2004 : Article 100 Definitions
Voltage (of a Circuit)
The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned.
FPN: Some systems, such as 3-phase 4-wire, single-phase 3-wire, and 3-wire direct-current, may have various circuits of various voltages.

Therefore a system of 150V or less would be a system where the circuit with the highest potential was 150V or less. 208/120 would be a 208V system with a 208V circuit and two 120V circuits.

 

[rbalex]

After talking with another electrician, he seems to recall the instructor in his NFPA 70e class telling him that if it is not stated phase to phase then it is assumed phase to ground. However he cannot find any documentation to back that up.

That's because it doesn't exist.

 

[rbalex]

Forum rule: Quickly answer the OP's question so we can go ahead and talk about what we want. :grin:

The OP "opened the door" when he said it was obvious the circuit couldn't be shut down.

A poorly designed ventilation system for a Div 1 location is not sufficent justification.

 

[scooby123]

Pfalcon, you have contradicted yourself. You stated your plant has a 120v subpanel (120v system) fed from a 480v main. So is this a typical single phase panel with 120/208 or 120/240? You called it a 120v system. By your definition, you should have called it a 208 or 240 system.

 

[pfalcon]

Pfalcon, you have contradicted yourself. You stated your plant has a 120v subpanel (120v system) fed from a 480v main. So is this a typical single phase panel with 120/208 or 120/240? You called it a 120v system. By your definition, you should have called it a 208 or 240 system.

To speak clearly:

The 480VAC three-phase is located in a main panel.
A 480 to 120 single phase transformer is used in the main panel.
120VAC single phase is run from the main panel to a remote secondary panel.

This is not 208/120 nor 240/120. It is 2-wire 120VAC single phase.

 

[scooby123]

Pfalcon, thanks for clarifying. That is an unusual system. I don?t think I've seen that particular application. So does the 120vac panel have only one buss or is there a jumper across both busses?

 

[LarryFine]

The original question was to define a "system" of less than 150 volts. Would this be phase to phase or phase to ground?

I guess what I'm trying to find out is did they make a rule for something that doesn't exist or are they drawing the line between 120/208-240 and 277/480.

I say it's the latter for both questions.


Added: If I'm wrong, please disregard the above statement.

 

[scooby123]

250.20 explains grounding of AC systems from 50 to 1000volts. 250.20(B)(1) "Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts." This leads me to believe that when referring to a system of less than or more than 150vac that it is L-G and not L-L. So therefore, a 120/208v 3 wire system or a 120v 2 wire system, like Pfalcon was referring to, would both be considered systems less than 150 volts. What do you think Larry?

 

[scooby123]

Pfalcon, what do the 120v 2-wire panels at your plant serve? I may have been mistaken in assuming you were talking about a distribution panel with branch circuits.

 

[pfalcon]

Pfalcon, thanks for clarifying. That is an unusual system. I don?t think I've seen that particular application. So does the 120vac panel have only one buss or is there a jumper across both busses?

Actually very common at industrial sites.

Pfalcon, what do the 120v 2-wire panels at your plant serve? I may have been mistaken in assuming you were talking about a distribution panel with branch circuits.

These are machine control panels not distribution panels.

So to both of the above, we are not attempting to balance the load across phases. These are literally single phase only. After the transformer there is no "buss" per se. Just terminals and conductors.

250.20 explains grounding of AC systems from 50 to 1000volts. 250.20(B)(1) "Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts." This leads me to believe that when referring to a system of less than or more than 150vac that it is L-G and not L-L. So therefore, a 120/208v 3 wire system or a 120v 2 wire system, like Pfalcon was referring to, would both be considered systems less than 150 volts. What do you think Larry?

I would disagree there. I read if I have a 208/120Y that I'm required to ground the neutral. I see nothing in it that says the Voltage (of a circuit) is any different than that defined in Article 100 of both the 70 and 70E.

 

[scooby123]

The illustration in exhibit 250.4 in the NEC handbook shows a 120/240v 3 wire and a 120v 2 wire system as examples of 250.20(B)(1). I have had a hard time finding a reference anywhere else describing a "system" more than or less than 150 volts. The definition of voltage is not in question. The question was, when defining a system less than 150v , is it measured L-L or L-G. It would appear that, according to 250.20(B)(1), it is measured L-G.

 

[LarryFine]

What do you think Larry?

I believe I've made my opinion clear: it refers to line to ground voltage.

If we're going to avoid line-to-line contact regardless, the line-to-ground voltage is the hazard.

 

[pfalcon]

The illustration in exhibit 250.4 in the NEC handbook shows a 120/240v 3 wire and a 120v 2 wire system as examples of 250.20(B)(1). I have had a hard time finding a reference anywhere else describing a "system" more than or less than 150 volts. The definition of voltage is not in question. The question was, when defining a system less than 150v , is it measured L-L or L-G. It would appear that, according to 250.20(B)(1), it is measured L-G.

Using 250.20(B)(1) as an example would be a misapplication. That section deals exclusively with grounding and therefore references everything to ground.

Article 100: Voltage (of a circuit) applies across the board and states any two conductors without regard to how they are used.

Annex D.7 uses phase-phase for arc-flash calculations.

And just found enough of ANSI C84.1 to answer the FPN under NOMINAL VOLTAGE

2 Definitions
2.1 system or power system: The connected system of power apparatus used to deliver electric power from the source to the utilization device. Portions of the system may be under different ownership, such as that of a supplier or a user.
2.2 system voltage terms. As used in this document, all voltages are rms phase-to-phase, except that the voltage following a slant-line is an rms phase-to-neutral voltage.
2.2.1 system voltage: The root-mean-square (rms) phase-to-phase voltage of a portion of an alternating-current electric system. Each system voltage pertains to a portion of the system that is bounded by transformers or utilization equipment.

So ANSI C84.1 uses phase-phase except where specifically notated to used phase-neutral. And system voltage is defined as phase-phase. Tough find. It's at the bottom of the content preview on their website.